Junction Temperature Control Method and Device for Inverter Switching Devices

Abstract

An embodiment of the present invention discloses a method and device for controlling the junction temperature of an inverter switching device. In the case of low voltage ride-through, the target reactive current for supporting the operation of the power grid is determined according to the measured power grid impedance parameters, and the minimum bus voltage of the inverter under the target reactive current is determined. Then, by controlling the output reactive current of the inverter to be the target reactive current, the stable operation of the power grid can be supported in the case of low voltage ride-through of the power grid. The bus voltage of the inverter is adjusted between the preset maximum bus voltage and the minimum bus voltage to determine the target bus voltage that satisfies the condition that the junction temperature of the switching device does not exceed the preset upper limit of the junction temperature. Subsequently, by controlling the inverter to operate at the target bus voltage, it is ensured that the junction temperature of the switching device in the inverter does not exceed the safe range, thereby improving the operation safety of the system.

Description

Technical Field

[0001] The present invention relates to the technical field of junction temperature control of power electronic devices, and particularly to a method and device for controlling the junction temperature of inverter switching devices. Background Art

[0002] In a photovoltaic power generation system, since grid-connected inverters are often applied in remote areas, their transmission lines are long and the resistance-inductance ratio is low, and the power grid generally exhibits weak grid characteristics. When the grid voltage drops, the inverter needs to provide support to the grid by outputting reactive current.

[0003] However, due to the impedance differences of the weak grid, the magnitudes of the reactive currents output by the inverter are different. Without effective control measures, the junction temperature of the inverter switching devices is likely to exceed the safe range, threatening the operation safety of the system. Summary of the Invention

[0004] The present invention provides a method and device for controlling the junction temperature of inverter switching devices, so as to optimize the inverter bus voltage, ensure that the junction temperature of the switching devices does not exceed the safe range, and improve the operation safety of the system.

[0005] According to one aspect of the present invention, a method for controlling the junction temperature of inverter switching devices is provided, including:

[0006] Measuring the grid impedance parameters in the case of low voltage ride-through;

[0007] Determining the target reactive current for supporting the grid operation according to the grid impedance parameters, and determining the minimum bus voltage of the inverter under the target reactive current;

[0008] Controlling the output reactive current of the inverter to be the target reactive current;

[0009] Adjusting the bus voltage of the inverter between the preset maximum bus voltage and the minimum bus voltage to determine the target bus voltage, and the target bus voltage satisfies that the junction temperature of the switching device does not exceed the preset junction temperature upper limit.

[0010] Optionally, the inverter is connected in a two-stage system; before adjusting the bus voltage of the inverter between the preset maximum bus voltage and the minimum bus voltage to determine the target bus voltage, it further includes:

[0011] Controlling the bus voltage of the inverter to be unchanged, adjusting the active output power of the inverter, and determining the active output power when the junction temperature of the switching device reaches the preset junction temperature upper limit as the maximum active output power;

[0012] Adjusting the bus voltage of the inverter between the preset maximum bus voltage and the minimum bus voltage to determine the target bus voltage includes:

[0013] When controlling the reactive current of the inverter to maintain the target reactive current and the active output power of the inverter is the maximum active output power, adjust the bus voltage of the inverter between the preset maximum bus voltage and the minimum bus voltage to determine the target bus voltage.

[0014] Optionally, when controlling the bus voltage of the inverter to remain unchanged and adjusting the active output power of the inverter, determining that the active output power when the junction temperature of the switching device reaches the preset upper limit of the junction temperature is the maximum active output power includes:

[0015] Control the bus voltage of the inverter to remain unchanged and gradually increase the active output power of the inverter from 0;

[0016] During the process of gradually increasing the active output power of the inverter, obtain the case temperature of each switching device in the inverter;

[0017] When the junction temperature of the switching device corresponding to the case temperature of at least one switching device reaches the preset upper limit of the junction temperature, determine the corresponding active output power as the maximum active output power.

[0018] Optionally, when controlling the reactive current of the inverter to maintain the target reactive current and the active output power of the inverter is the maximum active output power, adjusting the bus voltage of the inverter between the preset maximum bus voltage and the minimum bus voltage to determine the target bus voltage includes:

[0019] Control the reactive current of the inverter to maintain the target reactive current. When the active output power of the inverter is the maximum active output power, control the bus voltage to gradually decrease from the maximum bus voltage to the minimum bus voltage;

[0020] During the process of gradually decreasing the maximum bus voltage to the minimum bus voltage, obtain the case temperature of each switching device and determine the temperature margin between the maximum junction temperature and the preset upper limit of the junction temperature in each switching device according to the case temperature in each switching device;

[0021] Determine the bus voltage with the maximum temperature margin as the target bus voltage.

[0022] Optionally, after controlling the reactive current of the inverter to maintain the target reactive current, the active output power of the inverter is the maximum active output power, and adjusting the bus voltage of the inverter between the preset maximum bus voltage and the minimum bus voltage to determine the target bus voltage, it further includes:

[0023] Control the inverter to operate with the target reactive current, the maximum active output power and the target bus voltage;

[0024] Record the grid impedance parameters and the corresponding target reactive current, maximum active output power and target bus voltage as historical data;

[0025] After determining the grid impedance parameters, it further includes:

[0026] Determine whether the current grid impedance parameters match the grid impedance parameters in the historical data. If so, control the operation of the inverter according to the target reactive current, maximum active power output, and target bus voltage corresponding to the grid impedance parameters in the historical data;

[0027] If not, then perform the steps of determining the target reactive current for supporting the operation of the grid according to the grid impedance parameters and determining the minimum bus voltage of the inverter under the target reactive current.

[0028] Optionally, the inverter is connected in a single-stage system, and the single-stage system includes at least two photovoltaic strings; adjusting the bus voltage of the inverter between a preset maximum bus voltage and a minimum bus voltage to determine the target bus voltage includes:

[0029] Adjust the number of photovoltaic strings connected to the inverter;

[0030] At each number of photovoltaic strings connected to the inverter, adjust the bus voltage of the inverter between the preset maximum bus voltage and the minimum bus voltage, and obtain the junction temperature of each switching device. Determine the temperature margin according to the maximum junction temperature of the switching device and the preset upper limit of the junction temperature;

[0031] Determine the target number of photovoltaic strings connected to the inverter and the target bus voltage according to the temperature margin.

[0032] Optionally, adjusting the number of photovoltaic strings connected to the inverter includes:

[0033] Adjust the number of photovoltaic strings connected to the inverter in ascending order;

[0034] At each number of photovoltaic strings connected to the inverter, adjust the bus voltage of the inverter between the preset maximum bus voltage and the minimum bus voltage, and obtain the junction temperature of each switching device. Determine the temperature margin according to the maximum junction temperature of the switching device and the preset upper limit of the junction temperature, including:

[0035] At each number of photovoltaic strings connected to the inverter, control the bus voltage of the inverter to gradually decrease from the maximum bus voltage to the minimum bus voltage, and obtain the junction temperature of each switching device. Determine the temperature margin according to the maximum junction temperature of the switching device and the preset upper limit of the junction temperature;

[0036] Determining the target number of photovoltaic strings connected to an inverter and the target bus voltage according to the temperature margin, including: when i + 1 photovoltaic strings are connected to the inverter, if the temperature margin is equal to 0, determining i as the target number, and determining the bus voltage corresponding to the maximum temperature margin at the target number as the target bus voltage, where i is greater than or equal to 1 and less than the total number of photovoltaic strings.

[0037] Optionally, after determining the target number of photovoltaic strings connected to the inverter and the target bus voltage according to the temperature margin, it further includes:

[0038] Controlling the inverter to operate with the target reactive current, connecting the target number of photovoltaic strings and the target bus voltage;

[0039] Recording the grid impedance parameter and the corresponding target reactive current, target number, and target bus voltage of the grid impedance parameter as historical data;

[0040] After determining the grid impedance parameter, it further includes:

[0041] Determining whether the current grid impedance parameter matches the grid impedance parameter in the historical data. If so, controlling the inverter to operate according to the target reactive current, target number, and target bus voltage corresponding to the grid impedance parameter in the historical data;

[0042] If not, then perform the steps of determining the target reactive current for supporting the grid operation according to the grid impedance parameter and determining the minimum bus voltage of the inverter at the target reactive current.

[0043] Optionally, the grid impedance parameter includes grid impedance and grid inductive reactance; in the case of low voltage ride-through, measuring the grid impedance parameter includes:

[0044] In the case of low voltage ride-through, calculating the grid impedance by disturbing the active output power of the inverter and calculating the grid inductive reactance by the reactive output power of the inverter;

[0045] Determining the target reactive current for supporting the grid operation according to the grid impedance parameter, controlling the inverter output to reach the target reactive current, and determining the minimum bus voltage of the inverter at the target reactive current, including:

[0046] Determining the target reactive current according to the grid inductive reactance, the target voltage at the point of common coupling between the inverter and the grid, the grid voltage, and the grid angular frequency;

[0047] Determining the minimum bus voltage according to the target reactive current, the grid voltage, the grid inductive reactance, and the parameters of the filter inductance in the inverter.

[0048] According to another aspect of the present invention, there is provided a junction temperature control device for an inverter switching device, including:

[0049] A measurement module, configured to measure grid impedance parameters when a low voltage ride-through occurs;

[0050] A first determination module, configured to determine a target reactive current for supporting the operation of the grid according to the grid impedance parameters, and determine the minimum bus voltage of the inverter under the target reactive current;

[0051] A control module, configured to control the output reactive current of the inverter to be the target reactive current;

[0052] A second determination module, configured to adjust the bus voltage of the inverter between a preset maximum bus voltage and a minimum bus voltage to determine a target bus voltage, where the target bus voltage satisfies that the junction temperature of the switching device does not exceed a preset junction temperature upper limit.

[0053] The method and device for controlling the junction temperature of the switching device of the inverter according to the embodiments of the present invention can support the stable operation of the grid when a low voltage ride-through occurs by determining a target reactive current for supporting the operation of the grid according to the measured grid impedance parameters, determining the minimum bus voltage of the inverter under the target reactive current, and then controlling the output reactive current of the inverter to be the target reactive current. By adjusting the bus voltage of the inverter between the preset maximum bus voltage and the minimum bus voltage, a target bus voltage that satisfies that the junction temperature of the switching device does not exceed the preset junction temperature upper limit is determined. Subsequently, by controlling the inverter to operate at the target bus voltage, it is ensured that the junction temperature of the switching device in the inverter does not exceed the safe range, thereby improving the operation safety of the system.

[0054] It should be understood that the content described in this part is not intended to identify the key or important features of the embodiments of the present invention, nor is it used to limit the scope of the present invention. Other features of the present invention will become easily understood through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0056] Figure 1 is a flowchart of a method for controlling the junction temperature of a switching device of an inverter provided by an embodiment of the present invention;

[0057] Figure 2 is a schematic structural diagram of a two-stage system;

[0058] Figure 3It is a flowchart of another method for controlling the junction temperature of an inverter switching device provided by an embodiment of the present invention;

[0059] Figure 4 It is a schematic diagram of a single-phase topology structure of a Neutral Point Clamped (NPC) three-level inverter;

[0060] Figure 5 They are respectively the equivalent thermal impedance models of the switching device IGBT and the diode;

[0061] Figure 6 It is a schematic diagram of the junction temperature partition of the switching device in the inverter;

[0062] Figure 7 It is a specific flowchart for determining the maximum active output power;

[0063] Figure 8 It is a schematic diagram of the junction temperature curves of different switching devices in the NPC inverter under different active currents;

[0064] Figure 9 It is a specific flowchart for determining the target bus voltage;

[0065] Figure 10 It is a schematic diagram of the change in the junction temperature of the switching device under different bus voltages;

[0066] Figure 11 It is another schematic diagram of the change in the junction temperature of the switching device under different bus voltages;

[0067] Figure 12 It is a flowchart of another method for controlling the junction temperature of an inverter switching device provided by an embodiment of the present invention;

[0068] Figure 13 It is a schematic diagram of the structure of a single-stage system;

[0069] Figure 14 It is a flowchart of another method for controlling the junction temperature of an inverter switching device provided by an embodiment of the present invention;

[0070] Figure 15 It shows a schematic diagram of the change in the junction temperature of the switching device during the adjustment of the bus voltage in the case where only one photovoltaic string is connected to the inverter;

[0071] Figure 16 It shows another schematic diagram of the change in the junction temperature of the switching device during the adjustment of the bus voltage in the case where only one photovoltaic string is connected to the inverter;

[0072] Figure 17 It is a flowchart of another method for controlling the junction temperature of an inverter switching device provided by an embodiment of the present invention;

[0073] Figure 18 It is a waveform diagram for measuring grid impedance parameters of power disturbance.

[0074] Figure 19 It is the change of the junction temperature of the power switch device when disturbing the active current under the target reactive current of the inverter.

[0075] Figure 20 It shows a schematic diagram of the change of the overall system temperature margin when disturbing the bus voltage with the active current corresponding to the maximum active output power and the target reactive current both determined.

[0076] Figure 21 It shows another schematic diagram of the change of the overall system temperature margin when disturbing the bus voltage with the active current corresponding to the maximum active output power and the target reactive current both determined.

[0077] Figure 22 It shows a schematic diagram of the change of the overall system temperature margin when disturbing the bus voltage when the DC switch is closed to three paths.

[0078] Figure 23 It shows another schematic diagram of the change of the overall system temperature margin when disturbing the bus voltage when the DC switch is closed to three paths.

[0079] Figure 24 It is a schematic structural diagram of a junction temperature control device for an inverter switch device provided by an embodiment of the present invention. Detailed implementation manners

[0080] In order to enable those skilled in the art to better understand the solution of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0081] It should be noted that the terms "first", "second", etc. in the description, claims and above-mentioned drawings of the present invention are used to distinguish similar objects, and do not necessarily have to be used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged under appropriate circumstances, so that the embodiments of the present invention described here can be implemented in an order other than those illustrated or described here. In addition, the terms "comprising" and "having" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device comprising a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0082] Figure 1 FIG. is a flowchart of a method for controlling the junction temperature of an inverter switching device provided by an embodiment of the present invention. This embodiment is applicable to the case of controlling the reactive current output by the inverter to support the power grid during low voltage ride-through, and controlling the junction temperature of the switching device in the inverter by optimizing the bus voltage of the inverter, so that the junction temperature of the switching device does not exceed the preset upper limit of the junction temperature. This method can be executed by a junction temperature control device of the inverter switching device, and the junction temperature control device of the inverter switching device can be implemented in the form of hardware and / or software. Among them, the inverter can be connected to a DC power supply and the power grid respectively, and the DC power supply includes but is not limited to photovoltaic rectification and batteries, and the batteries include but are not limited to storage batteries and lithium batteries. As Figure 1 shown, the method includes:

[0083] S110. Measure the power grid impedance parameters in the case of low voltage ride-through.

[0084] In the case of low voltage ride-through, the power grid exhibits weak power grid characteristics, and the system enters the low voltage ride-through state. A weak power grid refers to a power grid that exhibits certain inductive characteristics under the combined action of non-linear loads and line impedances, and when the location of photovoltaic equipment connected to the power grid changes, the grid inductive reactance relative to the point of common coupling will also fluctuate. Such a power grid under non-ideal conditions is called a weak power grid. A weak power grid refers to a power grid that exhibits low stability and dynamic response capabilities under specific conditions, which is particularly obvious in distributed generation and remote areas. In this step, in the case of low voltage ride-through, the power grid impedance parameters are first measured. Optionally, the power grid impedance parameters are determined by disturbing the active output power and reactive output power of the inverter.

[0085] S120. Determine the target reactive current for supporting the operation of the power grid according to the power grid impedance parameters, and determine the minimum bus voltage of the inverter under the target reactive current.

[0086] Specifically, in the case of low voltage ride-through, to ensure the operation of the power grid, the inverter needs to provide reactive current. In this step, after the power grid impedance parameters are determined, the target reactive current for supporting the operation of the power grid is determined according to the power grid impedance parameters. Under this target reactive current, the common connection point between the inverter and the power grid can be restored to the voltage before the power grid experiences low voltage ride-through.

[0087] After determining the target reactive current, the minimum bus voltage of the inverter is determined according to the target reactive current. This minimum bus voltage needs to satisfy that there is no overmodulation under the Space Vector Pulse Width Modulation (SVPWM) modulation strategy, thereby ensuring the stability and security of the system.

[0088] S130. Control the output reactive current of the inverter to be the target reactive current.

[0089] In this step, controlling the output reactive current of the inverter to be equal to the target reactive current can support the stable operation of the power grid in the case of low voltage ride-through of the power grid.

[0090] S140. Adjust the bus voltage of the inverter between the preset maximum bus voltage and the minimum bus voltage to determine the target bus voltage, where the target bus voltage satisfies that the junction temperature of the switching device does not exceed the preset upper limit of the junction temperature.

[0091] Optionally, the preset maximum bus voltage is equal to the rated value of the bus voltage.

[0092] In this step, when adjusting the bus voltage between the maximum bus voltage and the minimum bus voltage, the junction temperature of the switching device in the inverter can be obtained in real time during this process. Since it is not easy to directly obtain the junction temperature of the switching device, in some embodiments, the case temperature of the switching device can be obtained, and the junction temperature of the switching device can be obtained according to the relationship between the junction temperature and the case temperature of the switching device. Optionally, among the voltage intervals with the maximum bus voltage and the minimum bus voltage as the interval endpoints, one of the bus voltages that satisfies that the junction temperatures of all switching devices in the inverter do not exceed the preset upper limit of the junction temperature is determined as the target bus voltage. Subsequently, by controlling the inverter to operate at the target bus voltage, it can be ensured that the junction temperature of the switching device in the inverter does not exceed the safe range, thereby improving the operating safety of the system.

[0093] Among them, adjusting the bus voltage between the maximum bus voltage and the minimum bus voltage can be gradually decreasing the bus voltage starting from the maximum bus voltage, or gradually increasing the bus voltage starting from the minimum bus voltage, or first gradually increasing from an intermediate bus voltage between the minimum bus voltage and the maximum bus voltage to the maximum bus voltage, and then gradually decreasing from the intermediate bus voltage to the minimum bus voltage. Adjusting the bus voltage between the maximum bus voltage and the minimum bus voltage can also be other adjustment methods, which are not specifically limited in the embodiments of the present invention.

[0094] It should be noted that the method for controlling the junction temperature of the inverter switching device in this embodiment can be applied to a two-stage system and a single-stage system. Optionally, when this method is applied to a two-stage system, during the process of adjusting the bus voltage of the inverter between the preset maximum bus voltage and the minimum bus voltage, the active output power of the inverter is controlled to be a fixed value, and this fixed value can be obtained in advance to ensure that the junction temperature of the switching device in the inverter does not exceed the preset upper limit of the junction temperature under the active output power, target bus voltage, and target reactive current corresponding to this fixed value. When this method is applied to a single-stage system, since the bus voltage output characteristic of the photovoltaic cell in the single-stage system changes with the change of the photovoltaic active output power, therefore, during the process of S140, the target active power of the inverter corresponding to the target bus voltage can also be obtained.

[0095] The method for controlling the junction temperature of the inverter switching device in this embodiment can support the stable operation of the power grid when a low voltage ride-through occurs. By determining the target reactive current for supporting the operation of the power grid according to the measured power grid impedance parameters and determining the minimum bus voltage of the inverter under the target reactive current when a low voltage ride-through occurs, and then controlling the output reactive current of the inverter to be the target reactive current. Adjusting the bus voltage of the inverter between the preset maximum bus voltage and the minimum bus voltage to determine the target bus voltage that satisfies the condition that the junction temperature of the switching device does not exceed the preset upper limit of the junction temperature. Subsequently, by controlling the inverter to operate at the target bus voltage, it is ensured that the junction temperature of the switching device in the inverter does not exceed the safe range, thereby improving the operation safety of the system.

[0096] Optionally, the inverter is connected in a two-stage system. Figure 2 is a schematic structural diagram of a two-stage system, refer to Figure 2, A two - stage system is a distributed system structure, usually composed of a front - stage and a rear - stage. In the front - stage, a DC / DC converter 10 (such as a boost circuit) is used to boost the DC voltage; in the rear - stage, an inverter 20 (such as a three - phase half - bridge inverter) is used to convert the DC voltage into an AC voltage and connect it to the power grid 40. That is to say, in the two - stage system, a DC / DC converter 10 is connected between the inverter 20 and the DC power supply 30, and the inverter 20 and the power grid 40 are connected at the point of common coupling (PCC). The DC power supply 30 of the two - stage system can be composed of a photovoltaic array, a storage battery, a lithium battery, etc., so as to achieve stable power supply under different working conditions. Figure 3 is a flowchart of another method for controlling the junction temperature of an inverter switching device provided by an embodiment of the present invention. Refer to Figure 3 , The method for controlling the junction temperature of the inverter switching device includes:

[0097] S210, Measure the grid impedance parameters in the case of low - voltage ride - through.

[0098] S220, Determine the target reactive current for supporting the operation of the power grid according to the grid impedance parameters, and determine the minimum bus voltage of the inverter under the target reactive current.

[0099] S230, Control the output reactive current of the inverter to be the target reactive current.

[0100] S240, Control the bus voltage of the inverter to remain unchanged, adjust the active output power of the inverter, and determine the active output power when the junction temperature of the switching device reaches the preset upper limit of the junction temperature as the maximum active output power.

[0101] Specifically, in this step, by controlling the output reactive current of the inverter to remain at the target reactive current and controlling the bus voltage of the inverter to remain unchanged. Optionally, control the bus voltage of the inverter to be the rated value of the bus voltage. That is, in the case where the output reactive current of the inverter is the target reactive current and the bus voltage of the inverter remains unchanged at the rated value, adjust the active output power of the inverter. At each active output power, respectively obtain the junction temperature of each switching device in the inverter. For example, determine the junction temperature of the switching device by obtaining the case temperature of the switching device, and then determine the active output power when the junction temperature of the switching device reaches the preset upper limit of the junction temperature, and determine this active output power as the maximum active output power. In this step, by determining the maximum active output power when the junction temperature of the switching device is the preset upper limit of the junction temperature, subsequent control of the inverter operation can control the inverter to output active power according to the maximum active output power, providing stronger power support for the power grid.

[0102] Optionally, S240 includes: controlling the bus voltage of the inverter to be constant and gradually increasing the active output power of the inverter from 0; during the process of gradually increasing the active output power of the inverter, obtaining the case temperatures of each switching device in the inverter; when the junction temperature of the switching device corresponding to the case temperature of at least one switching device reaches the preset upper limit of the junction temperature, determining the corresponding active output power as the maximum active output power.

[0103] Figure 4 is a schematic diagram of the single-phase topology structure of a Neutral Point Clamped (NPC) three-level inverter. Refer to Figure 4 , the switching intervals of the inverter include Insulate-Gate Bipolar Transistors (IGBTs) and diodes. Since the junction temperature of the switching device is difficult to directly measure, the case temperatures of the IGBT and the diode can be measured separately, and then the junction temperature of the IGBT can be determined based on the case temperature of the IGBT, the junction-case thermal impedance, and the loss, and the junction temperature of the diode can be determined based on the junction temperature of the diode, the junction-case thermal impedance, and the loss. Figure 5 are the equivalent thermal impedance models of the switching devices IGBT and diode respectively. According to the equivalent thermal impedance models, the junction temperatures of the IGBT and the diode can be calculated by Equations (1) and (2).

[0104] (1)

[0105] (2)

[0106] In the formula, T IJ , T DJ are the junction temperatures of the IGBT and the diode respectively, T Icase , T Dcase are the case temperatures of the IGBT and the diode respectively, P I , P D are the power loss values on the IGBT and the diode respectively, R IJC , R DJC are the junction-case thermal impedances of the IGBT and the diode respectively.

[0107] The power losses of the IGBT and the diode include two parts: switching loss and conduction loss. The conduction losses of the IGBT and the diode can be calculated by Equations (3) and (4) respectively. In the formula, R I , R D are the resistance values when the IGBT and the diode are conducting respectively, the saturation voltage drops V ce , V D and R I , R DThe relationship with the junction temperature can be obtained by referring to the device manual.

[0108] P I1 =V ce I c +I C 2 R I (3)

[0109] P D1 =V D I D +I D 2 R D (4)

[0110] In the formula, P I1 and P D1 are the conduction losses of the IGBT and the diode respectively, V ce and V D are the saturation voltage drops of the IGBT and the diode respectively, and I c and I D are the currents flowing through the IGBT and the diode respectively.

[0111] The switching losses of the IGBT and the diode can be calculated by Equations (5) and (6). The energies of the switching losses of the IGBT and the diode can be obtained by referring to the device manual.

[0112] P I2 =f sw E I (I C ) (5)

[0113] P D2 =f sw E D (I D ) (6)

[0114] In the formula, P I2 and P D2 are the switching losses of the IGBT and the diode, f sw is the switching frequency, and E I and E I are the switching loss energies of the IGBT and the diode respectively.

[0115] Figure 6 is a schematic diagram of the junction temperature partition of the switching device in the inverter. Refer to Figure 6, when the inverter is operating, the junction temperature of the switching device is set in three regions, namely the safe operating region, the warning operating region, and the fault operating region. When the junction temperature of the switching device is within the safe operating region and the warning operating region, the inverter can operate normally. When it is within the fault operating region, the inverter needs to stop working within a short time; otherwise, it will damage the power switching device and cause irreversible effects on the overall system. Optionally, in a two-stage system, the preset upper limit of the junction temperature can be equal to the upper limit of the temperature T in the warning operating region. 1 . In other alternative embodiments, to ensure the safe operation of the system, the preset upper limit of the junction temperature can also be less than the upper limit of the temperature T in the warning operating region. 1 , but greater than the upper limit of the temperature T in the safe operating region. 2 .

[0116] During the process of the active output power of the inverter gradually increasing, the junction temperatures of all switching devices show an increasing trend. Optionally, during the process of the active output power of the inverter gradually increasing, the active output power when the first switching device reaches the preset upper limit of the junction temperature is determined as the maximum active output power. In this way, it can be ensured that when the inverter is subsequently controlled to operate at the maximum active output power, the junction temperatures of all switching devices will not exceed the preset upper limit of the junction temperature, thereby ensuring the safe operation of the system.

[0117] Figure 7 is the specific determination flowchart of the maximum active output power. Refer to Figure 7 , the determination process of the maximum active output power includes: controlling the reactive output current of the inverter according to the target reactive current value, controlling the bus voltage of the inverter to be fixed, and setting the active output power of the inverter to 0; increasing the active output power of the inverter according to the first preset step; judging whether the maximum junction temperature among the switching devices in the inverter reaches the preset temperature upper limit; if so, determining the current active output power as the maximum active output power; if not, returning to execute the step of increasing the active output power of the inverter according to the first preset step.

[0118] Figure 8 is a schematic diagram of the junction temperature curves of different switching devices in an NPC inverter under different active currents. Among them Figure 8 only schematically shows one kind of junction temperature curve. Under different working conditions, the change of the junction temperature of the same switching device in the inverter can be different, and the junction temperature curves of the switching devices in different inverters can also be different. As Figure 4 shown, the NPC inverter includes IGBTs and diodes. According to the different functions of the IGBTs, they can be divided into two types, namely the first type of IGBT and the second type of IGBT, and the two types of IGBTs are respectively denoted as DCSW and ACSW; according to the different functions of the diodes, they are divided into three types, namely the first type of diode, the second type of diode, and the third type of diode, and the three types of diodes are respectively denoted as DCD, ACD, and NPD. Refer toFigure 4 and Figure 8 , taking the preset upper limit of the junction temperature as 110°C as an example, during the process of gradually increasing the active output power ( Figure 8 taking increasing the active current output by the inverter to increase the active output power as an example), among each switching device, the third type of diode NPD reaches 110°C first, and the active output power corresponding to the active current when the third type of diode NPD reaches 110°C first is determined as the maximum active output power.

[0119] S250. Control the reactive current of the inverter to maintain the target reactive current. When the active output power of the inverter is the maximum active output power, adjust the bus voltage of the inverter between the preset maximum bus voltage and the minimum bus voltage to determine the target bus voltage.

[0120] In this step, when controlling the output of the inverter to maintain the target reactive current and the maximum active output power, the target bus voltage is determined between the maximum bus voltage and the minimum bus voltage, and the target bus voltage satisfies that the junction temperature of each switching device in the inverter does not exceed the preset upper limit of the junction temperature. In this way, the target bus voltage of the inverter under the conditions of the target reactive current and the maximum active output power for supporting the grid operation can be determined, ensuring that in the event of a grid voltage dip, the inverter can support the grid operation and maintain the maximum output active power, avoiding the overload operation of the inverter by optimizing the bus voltage, reducing the thermal stress and losses of the switching devices, and enhancing the adaptability of the inverter under complex grid conditions.

[0121] Optionally, S250 includes: controlling the reactive current of the inverter to maintain the target reactive current. When the active output power of the inverter is the maximum active output power, control the bus voltage to gradually decrease from the maximum bus voltage to the minimum bus voltage; during the process of gradually decreasing the maximum bus voltage to the minimum bus voltage, obtain the case temperature of each switching device, and determine the temperature margin between the maximum junction temperature and the preset upper limit of the junction temperature in each switching device according to the case temperature of each switching device; determine the bus voltage when the temperature margin is the largest as the target bus voltage. In some embodiments, the bus voltage corresponding to other temperature margins greater than 0 can also be set as the target bus voltage.

[0122] Among them, the maximum junction temperature in each switching device is the maximum value of the junction temperatures corresponding to each switching device included in the inverter. Optionally, the temperature margin is equal to the difference between the preset upper limit of the junction temperature and the maximum junction temperature in each switching device. By determining the bus voltage corresponding to the largest temperature margin as the target bus voltage, it can be ensured that when the inverter operates at the target bus voltage, the margin between the junction temperature of each switching device and the preset upper limit of the junction temperature is relatively large, further reducing the thermal stress and losses of the switching devices.

[0123] In some embodiments, the bus voltage can also be controlled to gradually increase from the minimum bus voltage to the maximum bus voltage. During the process of the minimum bus voltage gradually increasing to the maximum bus voltage, the case temperature of each switching device is obtained, and the temperature margin is calculated. Then, the bus voltage at the time when the temperature margin is the largest is determined as the target bus voltage.

[0124] Combined with Figure 6 , in some embodiments, the difference between the maximum junction temperature of each switching device and the upper limit of the junction temperature in the safe operating area can also be determined as the temperature margin, and the bus voltage at the time when the temperature margin is the smallest is determined as the target bus voltage. Figure 9 is a specific flowchart for determining the target bus voltage. Refer to Figure 9 , the process for determining the target bus voltage includes: controlling the inverter to output the target reactive current and the maximum active output power, with the bus voltage being the preset maximum bus voltage; determining the junction temperature of the switching device based on the obtained case temperature of the switching device; selecting the difference between the maximum junction temperature and the upper limit of the junction temperature in the safe operating area to obtain the temperature margin; reducing the bus voltage, for example, the bus voltage can be reduced according to a second preset step size; determining whether the temperature margin decreases; if so, return to the step of reducing the bus voltage; if not, increase the bus voltage, and determine the bus voltage at the time when the temperature margin is the smallest as the target bus voltage.

[0125] Figure 10 is a schematic diagram of the change in the junction temperature of the switching device under different bus voltages. Figure 11 is another schematic diagram of the change in the junction temperature of the switching device under different bus voltages. During the process of the bus voltage decreasing from the maximum bus voltage to the minimum bus voltage, the change in the junction temperature of the switching device in the inverter mainly includes Figure 10 and Figure 11 two situations. In the case where the difference between the maximum junction temperature and the upper limit of the junction temperature in the safe operating area of the switching device is used as the temperature margin, for Figure 10 the shown change in the junction temperature, as the bus voltage decreases from the maximum bus voltage to the minimum bus voltage, the temperature margin first decreases and then increases; for Figure 11 the shown change in the junction temperature, as the bus voltage decreases from the maximum bus voltage to the minimum bus voltage, the temperature margin gradually decreases. For both change trends, by judging whether the temperature margin changes and decreases during the process of reducing the bus voltage, the bus voltage corresponding to the minimum temperature margin can be determined as the target bus voltage, thereby ensuring that the determined target bus voltage will not cause the inverter to operate overloaded and reducing the thermal stress and losses.

[0126] In the case where the temperature margin is equal to the difference between the preset upper limit of the junction temperature and the maximum junction temperature of each switching device, for Figure 10As shown in the junction temperature variation, during the process of the bus voltage decreasing from the maximum bus voltage to the minimum bus voltage, the temperature margin first increases and then decreases; for Figure 11 As shown in the junction temperature variation, during the process of the bus voltage decreasing from the maximum bus voltage to the minimum bus voltage, the temperature margin gradually increases. For both variation trends, during the process of the bus voltage decreasing, by judging whether the temperature margin is the maximum, the bus voltage corresponding to the maximum temperature margin can be determined as the target bus voltage, thereby ensuring that the determined target bus voltage will not cause the inverter to operate overloaded, and reducing thermal stress and losses.

[0127] Figure 12 is a flowchart of another method for controlling the junction temperature of an inverter switching device provided by an embodiment of the present invention. Referring to Figure 12 , this junction temperature control method includes:

[0128] S310. Measure the grid impedance parameters in the case of low voltage ride-through.

[0129] S320. Determine whether the current grid impedance parameters match the grid impedance parameters in the historical data.

[0130] Optionally, in the case that the absolute value of the difference between the current grid impedance parameters and the grid impedance parameters in the historical data is less than or equal to a set threshold, it is determined that the current grid impedance parameters match the grid impedance parameters in the historical data; otherwise, it is determined that the current grid impedance parameters do not match the grid impedance parameters in the historical data. In some embodiments, in the case that the current grid impedance parameters are equal to the grid impedance parameters in the historical data, it is determined that the current grid impedance parameters match the grid impedance parameters in the historical data; otherwise, it is determined that the current grid impedance parameters do not match the grid impedance parameters in the historical data.

[0131] If so, execute S330. Control the inverter to operate according to the target reactive current, maximum active output power, and target bus voltage corresponding to the grid impedance parameters in the historical data. That is, control the active output power connected to the inverter to be equal to the maximum active output power, and the reactive current output by the inverter to be the target reactive current, and the bus voltage of the inverter to be the target bus voltage.

[0132] If not, execute S340.

[0133] S340. Determine the target reactive current for supporting the grid operation according to the grid impedance parameters, and determine the minimum bus voltage of the inverter under the target reactive current.

[0134] S350. Control the output reactive current of the inverter to be the target reactive current.

[0135] S360. Keep the bus voltage of the inverter unchanged, adjust the active output power of the inverter, and determine the active output power when the junction temperature of the switching device reaches the preset upper limit of the junction temperature as the maximum active output power.

[0136] S370. Control the reactive current of the inverter to maintain the target reactive current. When the active output power of the inverter is the maximum active output power, adjust the bus voltage of the inverter between the preset maximum bus voltage and the minimum bus voltage to determine the target bus voltage.

[0137] S380. Control the inverter to operate with the target reactive current, the maximum active output power, and the target bus voltage. In this way, it can be ensured that in the case of low voltage ride-through, the inverter outputs the target reactive current to support the grid operation, outputs the maximum active output power under the condition of meeting the reactive power demand, provides stronger power support for the grid, and ensures that the junction temperature of the switching device under the target bus voltage does not exceed the limit.

[0138] S390. Record the grid impedance parameters and the corresponding target reactive current, maximum active output power, and target bus voltage as historical data.

[0139] Exemplarily, the historical data can be stored in a storage medium or in the cloud, which is convenient for subsequent reference when controlling the inverter in the case of low voltage ride-through of the grid.

[0140] In some embodiments, the inverter is connected in a single-stage system. Figure 13 is a schematic structural diagram of the single-stage system. Refer to Figure 13 . In the single-stage system, there is no DC / DC converter between the inverter 20 and the DC power supply 30, and the inverter 20 is connected to the grid 40 at the point of common coupling PCC. In the single-stage system, the DC power supply 30 is mainly a photovoltaic array, and the photovoltaic array includes at least two photovoltaic strings. Since the bus voltage output characteristic of the photovoltaic cells in the single-stage system changes with the change of the photovoltaic active output power, the method for controlling the junction temperature of the inverter switching device in the single-stage system is different from that in the two-stage system. Figure 14 is a flowchart of another method for controlling the junction temperature of the inverter switching device provided by an embodiment of the present invention. Refer to Figure 14 . The method for controlling the junction temperature of the inverter switching device includes:

[0141] S410. Measure the grid impedance parameters in the case of low voltage ride-through.

[0142] S420. Determine the target reactive current for supporting the grid operation according to the grid impedance parameters, and determine the minimum bus voltage of the inverter under the target reactive current.

[0143] S430. Control the output reactive current of the inverter to the target reactive current.

[0144] S440. Adjust the number of photovoltaic strings connected to the inverter.

[0145] Optionally, the single-stage system includes at least two photovoltaic strings. The photovoltaic strings are connected to the inverter through DC switches ( Figure 13 K1... Kn in which n is the total number of DC switches). By controlling the number of closed DC switches, the number of photovoltaic strings connected in parallel to the inverter can be controlled. Optionally, adjust the number of photovoltaic strings connected to the inverter in ascending order. Exemplarily, first control the number of photovoltaic strings connected to the inverter to be 1, and subsequently gradually increase the number of photovoltaic strings connected to the inverter. For example, each time one more photovoltaic string is connected to the inverter, that is, control the number of photovoltaic strings connected to the inverter in sequence as 1, 2, 3... n, where n is the total number of photovoltaic strings in the single-stage system. In this case, each DC switch corresponds to controlling the connection of one photovoltaic string to the inverter.

[0146] S450. At each number of photovoltaic strings connected to the inverter, adjust the bus voltage of the inverter between the preset maximum bus voltage and the minimum bus voltage, obtain the junction temperatures of each switching device, and determine the temperature margin according to the maximum junction temperature of the switching device and the preset junction temperature upper limit.

[0147] In some embodiments, the temperature margin is equal to the difference between the junction temperature upper limit and the maximum junction temperature. During the process of adjusting the number of photovoltaic strings connected to the inverter, control the inverter to output the target reactive current to support the operation of the power grid.

[0148] Optionally, S450 includes: at each number of photovoltaic strings connected to the inverter, control the bus voltage of the inverter to gradually decrease from the maximum bus voltage to the minimum bus voltage, obtain the junction temperatures of each switching device, and determine the temperature margin according to the maximum junction temperature of the switching device and the preset junction temperature upper limit.

[0149] Figure 15 A schematic diagram showing the change in the junction temperature of the switching device during the process of adjusting the bus voltage when only one photovoltaic string is connected to the inverter. Figure 16 Another schematic diagram showing the change in the junction temperature of the switching device during the process of adjusting the bus voltage when only one photovoltaic string is connected to the inverter. In different operating states, the change in the junction temperature of the same switching device can be different. In different inverters, the change in the junction temperature of the switching device can also be different. Combining Figure 4 and Figure 15, when the preset upper limit of the junction temperature is 110°C, during the entire adjustment process of the bus voltage, the maximum junction temperatures of all switching devices do not reach the preset upper limit of the junction temperature, and the temperature margins are all greater than 0. In this case, it is necessary to increase the number of photovoltaic strings connected to the inverter to determine the target number of photovoltaic strings connected to the inverter and the target bus voltage. Combining Figure 4 and Figure 16 , when the preset upper limit of the junction temperature is 110°C, during the entire adjustment process of the bus voltage, the maximum junction temperatures of all switching devices do not reach the preset upper limit of the junction temperature, and the temperature margins are all greater than 0. In this case, it is also necessary to increase the number of photovoltaic strings connected to the inverter to determine the target number of photovoltaic strings connected to the inverter and the target bus voltage.

[0150] S460. Determine the target number of photovoltaic strings connected to the inverter and the target bus voltage according to the temperature margin.

[0151] When the temperature margin is equal to the difference between the upper limit of the junction temperature and the maximum junction temperature, S460 may include: when i + 1 photovoltaic strings are connected to the inverter, if the temperature margin is equal to 0, determine i as the target number, and determine the bus voltage corresponding to the maximum temperature margin at the target number as the target bus voltage, where i is greater than or equal to 1 and less than the total number of photovoltaic strings. In this way, it can be ensured that under the determined target number, the active output power of the inverter is relatively large to provide stronger power support for the power grid. And at the target bus voltage of the target number, there is a relatively large temperature margin between the junction temperature of the switching device and the preset upper limit of the junction temperature, reducing thermal stress and losses.

[0152] Exemplarily, S460 specifically includes: controlling the number of photovoltaic strings connected in parallel to the inverter to be j, controlling the bus voltage of the inverter to decrease from the preset maximum bus voltage to the minimum bus voltage, and obtaining the junction temperatures of all switching devices; j is a positive integer greater than or equal to 1 and less than or equal to the total number of photovoltaic strings; determining the temperature margin between the maximum junction temperature and the preset upper limit of the junction temperature in each switching device according to the case temperature in each switching device; when the temperature margin is equal to 0, determining the target number of photovoltaic strings connected in parallel to the inverter as j - 1, and determining the bus voltage corresponding to the maximum temperature margin at the target number as the target bus voltage; when the temperature margin is greater than 0, set j = j + 1, and return to execute the step of controlling the number of photovoltaic strings connected in parallel to the inverter to be j.

[0153] Figure 17 is a flowchart of another method for controlling the junction temperature of an inverter switching device provided by an embodiment of the present invention. Refer to Figure 17 , optionally, the method for controlling the junction temperature of an inverter switching device includes:

[0154] S510. Measure the grid impedance parameters in the case of low voltage ride-through.

[0155] S520. Determine whether the current grid impedance parameters match the grid impedance parameters in the historical data.

[0156] If so, execute S530. Control the operation of the inverter according to the target reactive current, target number, and target bus voltage corresponding to the grid impedance parameters in the historical data. That is, control the number of photovoltaic strings connected to the inverter to be equal to the target number, and the reactive current output by the inverter to be the target reactive current, and the bus voltage of the inverter to be the target bus voltage.

[0157] If not, execute S540.

[0158] S540. Determine the target reactive current to support the operation of the grid according to the grid impedance parameters, and determine the minimum bus voltage of the inverter under the target reactive current.

[0159] S550. Control the output reactive current of the inverter to be the target reactive current.

[0160] S560. Adjust the number of photovoltaic strings connected to the inverter.

[0161] S570. At each number of photovoltaic strings connected to the inverter, adjust the bus voltage of the inverter between the preset maximum bus voltage and minimum bus voltage, and obtain the junction temperature of each switching device. Determine the temperature margin according to the maximum junction temperature of the switching device and the preset upper limit of the junction temperature.

[0162] S580. Determine the target number and target bus voltage of the photovoltaic strings connected to the inverter according to the temperature margin.

[0163] S590. Control the inverter to operate with the target reactive current, connect the target number of photovoltaic strings, and the target bus voltage. In this way, it can be ensured that in the case of low voltage ride-through, the inverter outputs the target reactive current to support the operation of the grid, controls the inverter to have a large active output power under the condition of meeting the reactive power demand, provides stronger power support for the grid, and ensures that the junction temperature of the switching device under the target bus voltage does not exceed the limit.

[0164] S600. Record the grid impedance parameters and the corresponding target reactive current, target number, and target bus voltage of the grid impedance parameters as historical data.

[0165] Based on the above embodiments, combined with Figure 2 and Figure 13 , optionally, the grid impedance parameters include the grid resistance R g and the grid inductive reactance L g; Among S110, S210, S310, S410, and S510, when low voltage ride-through occurs, measuring the grid impedance parameters includes: when low voltage ride-through occurs, calculating the grid impedance by disturbing the active output power of the inverter and calculating the grid inductive reactance by the reactive output power of the inverter.

[0166] Figure 18 is the waveform diagram of measuring the grid impedance parameters by power disturbance. Refer to Figure 18 , when low voltage ride-through occurs in the grid, first measure the grid impedance parameters, and the measurement schematic diagram is as Figure 18 shown. The measurement is mainly divided into two parts: measuring the grid impedance by active disturbance and measuring the grid inductive reactance by reactive disturbance. The calculation formulas are shown in Equations (7) and (8) respectively.

[0167] (7)

[0168] (8)

[0169] In the formula, R g is the grid impedance, L g is the grid inductive reactance, is the change in the d-axis of the grid voltage, is the change in the q-axis of the grid voltage, is the change in the d-axis of the grid current, is the change in the q-axis of the grid current, ω is the grid angular frequency, and its value is 2πf, where f is the grid frequency.

[0170] Optionally, S120, S220, S340, S420, S540, determine the target reactive current to support the grid operation according to the grid impedance parameters, control the inverter output to reach the target reactive current, and determine the minimum bus voltage of the inverter under the target reactive current, including: determining the target reactive current according to the grid inductive reactance, the target voltage of the common connection point between the inverter and the grid, the grid voltage, and the grid angular frequency; determining the minimum bus voltage according to the target reactive current, the grid voltage, the grid inductive reactance, and the parameters of the filter inductor in the inverter.

[0171] Specifically, since the resistance-inductance ratio in a weak grid is small, R g can be ignored in the calculation. After obtaining the grid impedance parameters, the reactive current required for the inverter to restore the common connection point, that is, the target reactive current, can be obtained through the formula.

[0172] I q = (V pcc - V g ) / ωL g (9)

[0173] In the formula, V pccis the target voltage to which the common connection point of the inverter and the grid needs to be restored, V g is the grid voltage.

[0174] The parameters of the grid-connected inverter filter inductor are represented by L x Then, according to Equation (10), when the SVPWM modulation strategy is adopted, the minimum value of the inverter bus voltage V bus can be obtained, which is also the minimum bus voltage.

[0175] V bus =(ω(L g +L x )I q +V g )*2 / 1.15 (10)

[0176] The following is an explanation of the method for controlling the junction temperature of the inverter switching device in the above embodiment of the present invention in combination with specific parameters.

[0177] For the parameters of the two-stage system embodiment: the rated bus voltage is 1500V, the rated grid-connected AC line voltage effective value is 800V, the grid-side phase voltage effective value is 461.89V, and its peak value is 461.89*1.414 = 653.11V. The low voltage ride-through depth is 0.5pu, the grid reactance is 4mH, and the inverter filter inductor is 500uH.

[0178] It can be calculated according to formula (9):

[0179] I q = (V pcc -V g ) / ωL g = (653.11 - 653.11*0.5) / 2π*50*0.004 = 260 (A)

[0180] It can be calculated according to formula (10):

[0181] V bus =(ω(L g +L x )I q +V g )*2 / 1.15 = (2π*50*(0.004 + 0.0005)*260 + (653.11*0.5))*2 / 1.15 = 1206.84(V)

[0182] It is calculated that the reactive current to be compensated (i.e., the target reactive current) is 260A, and the minimum bus voltage value is 1206V. In order to increase the margin, the sum of the calculated minimum bus voltage and the set margin voltage can be selected as the final minimum bus voltage. For example, the final minimum bus voltage is determined to be 1210V.Figure 19 When the inverter disturbs the active current power switch device junction temperature change under the target reactive current, where Figure 19 corresponds to Figure 8 It can be seen that as the active current changes, the maximum junction temperature of the switching device also changes. Finally, the active current corresponding to the maximum active output power is determined to be 21 A.

[0183] Figure 20 Fig. shows a schematic diagram of the change of the overall system temperature margin when disturbing the bus voltage with the active current corresponding to the maximum active output power and the target reactive current both determined. Taking the temperature margin equal to the difference between the upper limit of the junction temperature and the maximum junction temperature as an example, it can be seen from the figure that the overall system temperature margin is the largest when the bus voltage is 1287 V. Therefore, 1287 V is selected as the bus voltage under this low voltage ride-through depth and grid impedance parameters. Figure 21 Fig. shows another schematic diagram of the change of the overall system temperature margin when disturbing the bus voltage with the active current corresponding to the maximum active output power and the target reactive current both determined. It can be seen from the figure that as the bus voltage decreases, the temperature margin of the system gradually increases. Therefore, the minimum bus voltage value of 1210 V is selected as the bus voltage under this low voltage ride-through depth and grid impedance parameters.

[0184] Parameters of the single-stage system embodiment: The rated bus voltage is 1500 V, the rated grid-connected AC line voltage effective value is 800 V, the low voltage ride-through depth is 0.5 pu, the grid inductive reactance is 4 mH, the inverter filter inductance is 500 uH. When only one DC switch is closed, the PV cell terminal voltage is 1530 V, the maximum power point voltage is 1250 V, and the maximum power point current is 7.2 A. When multiple DC switches are closed, they form a parallel relationship, the voltage does not change, and the current increases in multiples. When the DC switch is closed to four circuits, when disturbing the bus voltage, the junction temperature of the power switch device exceeds the upper limit of the warning operating value (i.e., the preset upper limit of the junction temperature). Therefore, the previous state is selected, and the DC switch is closed to three circuits. At this time, the system temperature margin is disturbed when the bus voltage is disturbed. Figure 22 Fig. shows a schematic diagram of the change of the overall system temperature margin when disturbing the bus voltage when the DC switch is closed to three circuits. As Figure 22 shown, the state with the largest temperature margin is selected as the inverter system control parameter. At this time, its bus voltage is 1282 V, and the output active current is 21 A. Figure 23 Fig. shows another schematic diagram of the change of the overall system temperature margin when disturbing the bus voltage when the DC switch is closed to three circuits. The system temperature margin when disturbing the bus voltage is as Figure 23 shown. As the bus voltage decreases, the system temperature margin gradually increases. The lowest bus voltage of 1210 V is selected as the inverter system control parameter. At this time, its active current is 22.2 A.

[0185] It can be found from the above embodiments that the method for controlling the junction temperature of the inverter switching device during low-voltage ride-through provided by the present invention can not only output the corresponding reactive current as required to support the power grid, but also determine the maximum active current output capacity under this condition, and on this basis, perturb the bus voltage to determine the bus voltage value with the minimum junction temperature of the inverter, establish the curve of the bus voltage and the system temperature margin, reduce the overall loss of the inverter, and facilitate thermal management.

[0186] The embodiment of the present invention also provides a device for controlling the junction temperature of an inverter switching device. Figure 24 It is a schematic structural diagram of a device for controlling the junction temperature of an inverter switching device provided by an embodiment of the present invention. Refer to Figure 24 This junction temperature control device includes:

[0187] A measurement module 610, configured to measure the grid impedance parameters in the case of low-voltage ride-through.

[0188] A first determination module 620, configured to determine a target reactive current for supporting the operation of the power grid according to the grid impedance parameters, and determine the minimum bus voltage of the inverter under the target reactive current.

[0189] A control module 630, configured to control the output reactive current of the inverter to be the target reactive current.

[0190] A second determination module 640, configured to adjust the bus voltage of the inverter between a preset maximum bus voltage and the minimum bus voltage to determine a target bus voltage, where the target bus voltage satisfies that the junction temperature of the switching device does not exceed a preset junction temperature upper limit.

[0191] The device for controlling the junction temperature of the inverter switching device provided by the embodiment of the present invention can execute the method for controlling the junction temperature of the inverter switching device provided by any embodiment of the present invention, and has corresponding functional modules and beneficial effects for executing the method.

[0192] It should be understood that various forms of the processes shown above can be used, reordering, adding or deleting steps. For example, the steps described in the present invention can be executed in parallel, sequentially, or in different orders, as long as the desired results of the technical solution of the present invention can be achieved, and no limitation is made herein.

[0193] The above specific embodiments do not constitute a limitation to the protection scope of the present invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for controlling junction temperature of an inverter switch device, characterized in that: include: In case of low voltage ride-through, measure the grid impedance parameters; Determine a target reactive current supporting the operation of the grid according to the grid impedance parameter, and determine a minimum bus voltage of the inverter under the target reactive current; the minimum bus voltage satisfies that no overmodulation occurs under the modulation strategy of space vector pulse width modulation; Controlling the output reactive current of the inverter to be the target reactive current; Adjusting the bus voltage of the inverter between a preset maximum bus voltage and the minimum bus voltage to determine a target bus voltage, wherein the target bus voltage satisfies that the junction temperature of the switching device does not exceed a preset junction temperature upper limit; The inverter is connected in a two-stage system; Before adjusting the bus voltage of the inverter between a preset maximum bus voltage and the minimum bus voltage to determine a target bus voltage, the method further includes: Controlling the bus voltage of the inverter to remain unchanged, adjusting the active output power of the inverter, and determining that the active output power when the junction temperature of the switching device reaches the preset junction temperature upper limit is the maximum active output power; The step of adjusting the bus voltage of the inverter between a preset maximum bus voltage and a preset minimum bus voltage to determine a target bus voltage includes: Controlling the reactive current of the inverter to maintain the target reactive current, and when the active output power of the inverter is the maximum active output power, adjusting the bus voltage of the inverter between a preset maximum bus voltage and a minimum bus voltage to determine the target bus voltage; Under the condition that the reactive current of the inverter is controlled to maintain the target reactive current and the active output power of the inverter is the maximum active output power, after adjusting the bus voltage of the inverter between the preset maximum bus voltage and the minimum bus voltage to determine the target bus voltage, the method further includes: Controlling the inverter to operate with the target reactive current, the maximum active output power and the target bus voltage; Recording the grid impedance parameter and the target reactive current, the maximum active output power and the target bus voltage corresponding to the grid impedance parameter as historical data; After determining the grid impedance parameter, the method further includes: Determine whether the current grid impedance parameter matches the grid impedance parameter in the historical data, and if so, control the inverter operation according to the target reactive current, the maximum active output power and the target bus voltage corresponding to the grid impedance parameter in the historical data; If not, the steps of determining a target reactive current for supporting grid operation according to the grid impedance parameter and determining a minimum bus voltage of the inverter under the target reactive current are performed.

2. The method for controlling junction temperature of an inverter switch device according to claim 1, characterized in that: The controlling the bus voltage of the inverter to remain unchanged, adjusting the active output power of the inverter, and determining that the active output power when the junction temperature of the switching device reaches the preset junction temperature upper limit is the maximum active output power, includes: Controlling the bus voltage of the inverter to remain unchanged, and gradually increasing the active output power of the inverter from 0; In the process of gradually increasing the active output power of the inverter, obtaining the shell temperature of each of the switching devices in the inverter; When the junction temperature of the switching device corresponding to the shell temperature of at least one of the switching devices reaches the preset junction temperature upper limit, the corresponding active output power is determined as the maximum active output power.

3. The method for controlling junction temperature of an inverter switch device according to claim 1, characterized in that: The controlling the reactive current of the inverter to maintain the target reactive current and the active output power of the inverter being the maximum active output power, adjusting the bus voltage of the inverter between a preset maximum bus voltage and a minimum bus voltage to determine a target bus voltage, comprises: controlling the reactive current of the inverter to maintain the target reactive current, and when the active output power of the inverter is the maximum active output power, controlling the bus voltage to gradually decrease from the maximum bus voltage to the minimum bus voltage; In the process of the maximum bus voltage gradually decreasing to the minimum bus voltage, the shell temperature of each of the switching devices is acquired, and the temperature margin between the maximum junction temperature of each of the switching devices and the preset junction temperature upper limit is determined according to the shell temperature of each of the switching devices; The bus voltage when the temperature margin is maximum is determined as the target bus voltage.

4. The method for controlling junction temperature of an inverter switch device according to claim 1, characterized in that: The grid impedance parameters include grid impedance and grid inductive reactance; The measuring of the grid impedance parameters in the case of low voltage ride through includes: In the event of low voltage ride-through, the grid impedance is calculated by disturbing the active output power of the inverter, and the grid inductive reactance is calculated by the reactive output power of the inverter; The step of determining a target reactive current for supporting grid operation according to the grid impedance parameter, controlling the inverter output to reach the target reactive current, and determining a minimum bus voltage of the inverter under the target reactive current includes: Determining the target reactive current according to the grid inductive reactance, the target voltage of the common connection point between the inverter and the grid, the grid voltage and the grid angular frequency; The minimum bus voltage is determined according to the target reactive current, the grid voltage, the grid inductance, and parameters of the filter inductance in the inverter.

5. A method for controlling junction temperature of an inverter switch device, characterized in that: include: In case of low voltage ride-through, measure the grid impedance parameters; Determine a target reactive current supporting the operation of the grid according to the grid impedance parameter, and determine a minimum bus voltage of the inverter under the target reactive current; the minimum bus voltage satisfies that no overmodulation occurs under the modulation strategy of space vector pulse width modulation; Controlling the output reactive current of the inverter to be the target reactive current; Adjusting the bus voltage of the inverter between a preset maximum bus voltage and the minimum bus voltage to determine a target bus voltage, wherein the target bus voltage satisfies that the junction temperature of the switching device does not exceed a preset junction temperature upper limit; The inverter is connected in a single-stage system, and the single-stage system includes at least two photovoltaic strings; The step of adjusting the bus voltage of the inverter between a preset maximum bus voltage and a preset minimum bus voltage to determine a target bus voltage includes: adjusting the number of photovoltaic strings connected to the inverter; At each number of photovoltaic strings connected to the inverter, the bus voltage of the inverter is adjusted between a preset maximum bus voltage and the minimum bus voltage, and the junction temperature of each of the switching devices is obtained, and the temperature margin is determined according to the maximum junction temperature of the switching device and the preset junction temperature upper limit; determining a target number of photovoltaic strings connected to the inverter and the target bus voltage according to the temperature margin; After determining the target number of photovoltaic strings connected to the inverter and the target bus voltage according to the temperature margin, the method further includes: Controlling the inverter to operate at the target reactive current, connecting the target number of photovoltaic strings and the target bus voltage; Recording the grid impedance parameter and the target reactive current, the target quantity and the target bus voltage corresponding to the grid impedance parameter as historical data; After determining the grid impedance parameter, the method further includes: Determine whether the current grid impedance parameter matches the grid impedance parameter in the historical data, and if so, control the inverter operation according to the target reactive current, the target quantity, and the target bus voltage corresponding to the grid impedance parameter in the historical data; If not, the steps of determining a target reactive current for supporting grid operation according to the grid impedance parameter and determining a minimum bus voltage of the inverter under the target reactive current are performed.

6. The method for controlling junction temperature of an inverter switch device according to claim 5, characterized in that: The adjusting the number of photovoltaic strings connected to the inverter comprises: Adjusting the number of photovoltaic strings connected to the inverter in ascending order; The step of adjusting the bus voltage of the inverter between a preset maximum bus voltage and a preset minimum bus voltage at each number of photovoltaic strings connected to the inverter, acquiring the junction temperature of each switching device, and determining the temperature margin according to the maximum junction temperature of the switching device and the preset junction temperature upper limit comprises: At each number of photovoltaic strings connected to the inverter, the bus voltage of the inverter is controlled to gradually decrease from the maximum bus voltage to the minimum bus voltage, and the junction temperature of each switching device is obtained, and the temperature margin is determined according to the maximum junction temperature of the switching device and the preset junction temperature upper limit; The method of determining the target number of photovoltaic strings connected to the inverter and the target bus voltage according to the temperature margin includes: when i+1 photovoltaic strings are connected to the inverter, if the temperature margin is equal to 0, i is determined as the target number, and the bus voltage with the maximum temperature margin corresponding to the target number is determined as the target bus voltage, wherein i is greater than or equal to 1 and less than the total number of the photovoltaic strings.

7. The method for controlling junction temperature of an inverter switch device according to claim 5, characterized in that: The grid impedance parameters include grid impedance and grid inductance; when low voltage ride through occurs, measuring the grid impedance parameters includes: In the event of low voltage ride-through, the grid impedance is calculated by disturbing the active output power of the inverter, and the grid inductive reactance is calculated by the reactive output power of the inverter; The step of determining a target reactive current for supporting grid operation according to the grid impedance parameter, controlling the inverter output to reach the target reactive current, and determining a minimum bus voltage of the inverter under the target reactive current includes: Determining the target reactive current according to the grid inductive reactance, the target voltage of the common connection point between the inverter and the grid, the grid voltage and the grid angular frequency; The minimum bus voltage is determined according to the target reactive current, the grid voltage, the grid inductance, and parameters of the filter inductance in the inverter.

8. A junction temperature control device for an inverter switch device, characterized in that: include: A measurement module is used to measure grid impedance parameters in the event of low voltage ride-through; A first determination module is used to determine a target reactive current supporting the operation of the power grid according to the power grid impedance parameter, and to determine a minimum bus voltage of the inverter under the target reactive current; the minimum bus voltage satisfies that no overmodulation occurs under the modulation strategy of space vector pulse width modulation; A control module, used for controlling the output reactive current of the inverter to be the target reactive current; A second determination module adjusts the bus voltage of the inverter between a preset maximum bus voltage and the minimum bus voltage to determine a target bus voltage, wherein the target bus voltage satisfies that the junction temperature of the switching device does not exceed a preset junction temperature upper limit; The inverter is connected in a two-stage system; Before adjusting the bus voltage of the inverter between a preset maximum bus voltage and the minimum bus voltage to determine a target bus voltage, the method further includes: Controlling the bus voltage of the inverter to remain unchanged, adjusting the active output power of the inverter, and determining that the active output power when the junction temperature of the switching device reaches the preset junction temperature upper limit is the maximum active output power; The step of adjusting the bus voltage of the inverter between a preset maximum bus voltage and a preset minimum bus voltage to determine a target bus voltage includes: Controlling the reactive current of the inverter to maintain the target reactive current, and when the active output power of the inverter is the maximum active output power, adjusting the bus voltage of the inverter between a preset maximum bus voltage and a minimum bus voltage to determine the target bus voltage; Under the condition that the reactive current of the inverter is controlled to maintain the target reactive current and the active output power of the inverter is the maximum active output power, after adjusting the bus voltage of the inverter between the preset maximum bus voltage and the minimum bus voltage to determine the target bus voltage, the method further includes: Controlling the inverter to operate with the target reactive current, the maximum active output power and the target bus voltage; Recording the grid impedance parameter and the target reactive current, the maximum active output power and the target bus voltage corresponding to the grid impedance parameter as historical data; After determining the grid impedance parameter, the method further includes: Determine whether the current grid impedance parameter matches the grid impedance parameter in the historical data, and if so, control the inverter operation according to the target reactive current, the maximum active output power and the target bus voltage corresponding to the grid impedance parameter in the historical data; If not, the steps of determining a target reactive current for supporting grid operation according to the grid impedance parameter and determining a minimum bus voltage of the inverter under the target reactive current are performed.

9. A junction temperature control device for an inverter switch device, characterized in that: include: A measurement module is used to measure grid impedance parameters in the event of low voltage ride-through; A first determination module is used to determine a target reactive current supporting the operation of the power grid according to the power grid impedance parameter, and to determine a minimum bus voltage of the inverter under the target reactive current; the minimum bus voltage satisfies that no overmodulation occurs under the modulation strategy of space vector pulse width modulation; A control module, used for controlling the output reactive current of the inverter to be the target reactive current; A second determination module adjusts the bus voltage of the inverter between a preset maximum bus voltage and the minimum bus voltage to determine a target bus voltage, wherein the target bus voltage satisfies that the junction temperature of the switching device does not exceed a preset junction temperature upper limit; The inverter is connected in a single-stage system, and the single-stage system includes at least two photovoltaic strings; The step of adjusting the bus voltage of the inverter between a preset maximum bus voltage and a preset minimum bus voltage to determine a target bus voltage includes: adjusting the number of photovoltaic strings connected to the inverter; At each number of photovoltaic strings connected to the inverter, the bus voltage of the inverter is adjusted between a preset maximum bus voltage and the minimum bus voltage, and the junction temperature of each of the switching devices is obtained, and the temperature margin is determined according to the maximum junction temperature of the switching device and the preset junction temperature upper limit; determining a target number of photovoltaic strings connected to the inverter and the target bus voltage according to the temperature margin; After determining the target number of photovoltaic strings connected to the inverter and the target bus voltage according to the temperature margin, the method further includes: Controlling the inverter to operate at the target reactive current, connecting the target number of photovoltaic strings and the target bus voltage; Recording the grid impedance parameter and the target reactive current, the target quantity and the target bus voltage corresponding to the grid impedance parameter as historical data; After determining the grid impedance parameter, the method further includes: Determine whether the current grid impedance parameter matches the grid impedance parameter in the historical data, and if so, control the inverter operation according to the target reactive current, the target quantity, and the target bus voltage corresponding to the grid impedance parameter in the historical data; If not, the steps of determining a target reactive current for supporting grid operation according to the grid impedance parameter and determining a minimum bus voltage of the inverter under the target reactive current are performed.

Images